Chart: Additional Flow Required When Mixing E85 with Gas

Setting aside the challenges of getting consistent E85 blends for a moment.
I made some calculations for the extra flow required based on desired octane for gas/E85 blends.
If you're already on a injector high duty cycle and fuel pressure with your system, this might be handy.

Octane doesn't really work that way. Also, Octane doesn't really mean much especially for fuels over 100. Octane just measures knock resistance on some old tractor engines in a test lab, does not necessarily mean any particular performance on a modern turbocharged engine.

Bathtub chemistry, mixing fuels.

The big issue was cast aside in the first sentance.
"Setting aside the challenges of getting consistent E85 blends for a moment."

What you get out of the pump is not governed in any way other than a minimum amount of gas in it so folks wont drink it.

It's not worth spilling in your tank for performance reasons.

Real E85 comes out of a sealed drum. (and doesn't cold start well)
That tells me that the pump E85 has a lot of gas in it, may not even be the best quality gas since the Ethanol may prop it up some.

Unless it exceptionally warm overnight the drag cars need to be primed with gas to start the first time each day.

Not sure the backing on ^^^^, but my experience here running straight pump E85 over the last 6 years is far different.

Yes, there's a noticeable variance in the blend, even at the same pump.
Yes, it usually requires 2 or 3 tries to start first time in the morning, starts every time though. That's also with no cold start injector to help out.
Yes, there's a BIG performance improvement with pump grade E85, at least here in Cali. We have crap gas anyway, but throwing in a 60/40 mix of pump and E85 allows for a good bit more power since you can actually run the stock timing map, rather than going full retard. Going straight E85 and you're talking a big bump once you start adding in a little timing.

Sure, pump E85 isn't what sealed drum E85 is, that's a given. It's definitely not in the 'not worth spilling in your tank' though, even on a bone stock ride, if you do it in moderation.

After I put the flex fuel sensor on my 245, I did see some variability from tank to tank. From a low of about 40% to a high of about 80% (although you do need to calibrate the flex fuel sensor with a known fuel in order to get a 'real' and not 'relative' number, and I never got around to doing that).

Based on that, and anecdotes read online (some people say they've gotten as low as 10% from an E85 pump - that's plain old regular) I would agree that 'blind' tuning for E85 is a bit risky using pump E85. Blind meaning you don't have a flex fuel sensor and an ECU setup that does flex fuel functions, and you are tuning things fairly close to the limit assuming you always get a nice strong batch of E85. A random batch of rogue E10 from a pump could be a bad thing in that case.

We don't get that kind of variance around here. ~73 to 85+ % just about year round. If you pay attention to your car at all you'll know if you got a rogue batch of fuel before you put the beans to it, and if you actually got E10 it'd probably drown the spark before you ever got to a critical point (but then, that's the risk of flying blind eh).
Couple other anecdotal things:

If you're tuning on the ragged edge of pump anything, you're playing with fire. There's equally no promise that 93 is in fact 93, but you won't get wonky AFR readings to warn you there.

The volume difference between e85 and pump gas is higher than 30%, I think its probably closer to 54% higher (for straight E85, not e50), however blending it with regular gas moves that number around.

In practice, partial mixes of e85 have known performance qualities and this has been documented for quite some time. You can get a lot of the knock resistance from a blend without hammering your fuel system.


Cold start is only difficult if you don't tune for it, and by cold start I mean sub 40 degree coolant/oil temps, and sub 40 air temps. And again, only due to tuning (recently tested this on a customer car that had been sitting for about a week and temps dropped below freezing. good time to give it a shot). Pump gas cold start is easier to tune because the span of volumes is generally tighter and not as exponential.

There are other fringe benefits with E85 that don't show up in the octane rating, for example latent heat of evaporation. Price relative to other performance fuels. It does clean things well. It also corrodes stuff nicely esp if you let it sit for long periods of time..

I should make clear that the thought isn't to "tune with/to" those numbers. Really I'm looking to create overhead (10.7:1 compression) and get a very rough idea of whether the hardware I have is likely to flow enough.

linuxman51 said:

In practice, partial mixes of e85 have known performance qualities and this has been documented for quite some time. You can get a lot of the knock resistance from a blend without hammering your fuel system.

There are other fringe benefits with E85 that don't show up in the octane rating, for example latent heat of evaporation. Price relative to other performance fuels.

Click to expand...

That's prettymuch my line of thinking. Hedge my bets if you will.